OMEGAMIN
OMEGAMIN = [real]
Default: OMEGAMIN = minimum transition energy (0.05 for metals)
Description: OMEGAMAX minimum frequency in the frequency grid.
For the frequency grid along the real and imaginary axis sophisticated schemes are used, which are based on simple model functions for the macroscopic dielectric function. The grid spacing is dense up to roughly 1.3*OMEGAMAX and becomes coarser for larger frequencies. The default value for OMEGAMAX is either determined by the outermost node in the dielectric function (corresponding to a singularity in the inverse of the dielectric function) or the energy difference between the valence band minimum and the conduction band minimum. The larger of these two values is used. Except for pseudopotentials with deep lying core states, OMEGAMAX is usually determined by the node in the dielectric function.
For ACFDT calculations, only OMEGAMIN and OMEGATL determine the frequency grid (using a minimax algorithm).
The defaults have been carefully tested, and it is recommended to leave them unmodified, whenever possible. The grid should be solely controlled by NOMEGA. The only other value that can be modified is the complex shift CSHIFT. In principle, CSHIFT should NOT be chosen independently of NOMEGA and OMEGAMAX: e.g. for less dense grids (smaller NOMEGA) the complex shift must be accordingly increased. The default for CSHIFT has been chosen such that the calculations are converged to 10 meV with respect to NOMEGA: i.e. if CSHIFT is kept constant and NOMEGA is increased, the QP shifts should not change by more than 10 meV; at least for LSPECTRAL = .TRUE.. This was the case for the considered test materials. For LSPECTRAL = .FALSE. this does not apply. In this case it is recommended to set CSHIFT manually and to perform careful convergence tests.
For LSPECTRAL = .TRUE. independent convergence tests with respect to NOMEGA and CSHIFT are usually not required, and it should be sufficient to control the technical parameters via the single parameter NOMEGA. Also note that too large values for NOMEGA in combination with coarse k-point grids can cause a decrease in precision (see NOMEGA).